Add South migration for subscriber

Running 'syncdb' on a production database is a dangerous thing. We should
provide a South migration by default instead.
Merge pull request #10 from achiang/master

Add South migration for subscriber
Merge pull request #12 from IlianIliev/master

Bad import fix
Removed useless files
Big update

- Python3 support
- callback is a CBV
- no more urllib2
- removed the need for django.contrib.sites to be installed
- much better support of the PubSubHubbub spec, versions 0.3 and 0.4.
Added links to the specs
Coverage instructions
Made the secret blankable instead of null=True
Fixe name -- it's hub, not callback
Excluded tests from installed files
Fixed error message that can be thrown on unsubscriptions as well
Documented how to fake 1st migration
Truncated the topic in the admin if it's too long
Removed hub doc, inexisting
Re-added has_expired to the admin
Updated changelog (typos, 0.4 was missing)
It doesn't anymore
Removed hub mention, promote 0.4 compatibility
Test for hub credentials
Polish the docs